Mobility of a wireless communication device in a wireless communication system typically involves re-selection and handover procedures (commonly referred to herein as cell switch procedures). Re-selection is initiated by the wireless device and is typically applied when the device is in modes like e.g. idle mode or CELL_FACH, while handover is initiated by the system network based on measurement reports from the wireless device and is typically applied when the device is in modes like e.g. CELL_DCH.
In many system scenarios different radio access technologies (RAT) may co-exist, and cell switches may occur between cells both intra-RAT and inter-RAT. Examples of different radio access technologies are WCDMA (UTRA-FDD), LTE (E-UTRA) and TD-SCDMA (UTRA-TDD, LCR) (all according to the 3GPP specifications). In some scenarios, cell switches may be frequent due to e.g. high speed of the device, the device requesting various services (provided by different RATs), hierarchical or heterogeneous cell structures, etc.
Mobility management (both inter-RAT and intra-RAT) commonly involves cell switch parameters (such as e.g. Treselection, Time-To-Trigger and Qhyst according to the 3GPP specifications) and mobility parameters (such as e.g. Tcr and Ncr according to the 3GPP specifications). The cell switch parameters are typically used to determine when to perform a cell switch and the mobility parameters are typically used to determine how fast the wireless communication device is moving.
For the purpose of this disclosure, the cell switch parameters (typically provided to a device by the network) will function according to any suitable known or future method, and will thus not be elaborated on. It is noted, however, that one or more of the cell switch parameters may be scaled in the wireless communication device based on an estimated speed of the device. For example, a device moving at high speed may require a lower Treselection value than the default value to not lose service. In one example, a scaling factor is applied that has different values depending on which mobility state the device is in. Various standards may have different mobility state terminology (e.g. low/medium/high or normal/high).
The mobility state may also be used to control if the device prefers a macro or micro/pico cell of a hierarchical cell structure (HCS). Typically, a device in high mobility should not camp of micro/pico cells, while a device in normal/low mobility should camp on a cell having a highest HCS priority.
Which mobility state a device is in may be determined based on one or more of the mobility parameters. The mobility parameters are typically related to the cell switch frequency. For example, a device may count the number of cell switches it performs during a time window (e.g. Tcr) and compare this number to one or more thresholds (e.g. Ncr) to determine which mobility state it is in (a high number of cell switches typically indicates high mobility and vice versa).
However, there is a problem with directly applying the cell switch count above when cell switches between cells of different sizes take place. This is because the count value will differ much between a scenario where there are many small sized cells and a scenario where there are mainly large sized cells, even if the device is moving with the same speed. Hence, the corresponding mobility states determined in these two scenarios will not both accurately describe the factual conditions.
This is particularly seen in heterogeneous networks (use of multiple types of access nodes in a wireless network). An example of a heterogeneous network is where a Wide Area Network use macrocells, picocells, and/or femtocells in order to offer wireless coverage in an environment with a wide variety of wireless coverage zones, ranging from an open outdoor environment to office buildings, homes, and underground areas. In some employments, a heterogeneous network could be as a network with complex interoperation between macrocell, small cell, and in some cases WiFi network elements used together to provide a mosaic of coverage, with handoff capability between network elements.
A similar problem arises in a network situation where the cell sizes are dynamically adaptable (e.g. autonomous or self-optimizing networks). In such situations the size of a cell may change over time based on e.g. current traffic load and capacity of the cell and its neighbors. Since the cell sizes are dynamic, it will not be appropriate to apply the above procedures (cell switch count and cell switch parameters) in such networks.
This problem has been observed in US 2011/0021201, where a method is disclosed of determining a mobility state based on the number of cell reselections and a size of the cell to be reselected. In that disclosure, a reselection to a cell having a small cell size is not counted in the number of cell reselections. Thus, micro cells will not affect the mobility state determination. However, this solution will only be effective in a hierarchical cell structure system.
“Enhanced mobility state detection based mobility optimization for femto cells in LTE and LTE-advanced networks”, by Lei Yixue and Zhang Yongsheng, 2011 IET International Conference on Communication Technology and Application, discloses mobility management for femto cells where it is assumed that UE can distinguish the type of neighboring cell, i.e. femto cell or macro cell. The cell crossing counting for femto cell is weighted before adding to the total cell crossing. However, this solution assumes that the average cell size of macro cell and femto cell is known.
Another general problem is that for inter-RAT cell switches, the mobility state and/or the cell switch counter is commonly reset after the inter-RAT cell switch. For example, when switching from WCDMA to LTE, the mobility state is set to a default value indicating normal mobility regardless if the device was in high mobility in WCDMA. Furthermore, the cell switch counter is reset to zero, so a number of new cell switches first have to be performed within a time limit for the device to transform to a high mobility state. Until then, even if the device is moving at high speed, it is considered to be in normal mobility. This may lead to non-optimal performance for the device in LTE and a corresponding decision to switch back to WCDMA.
Typically, the cell sizes in a network depend on factors like requirements on the number of users to be supported per cell and other capacity requirements. Since the capacity planning requirements could be different for different RATs, the cell sizes could be different as well. Thus, the problem with directly applying the cell switch count when cell switches between cells of different sizes take place is particularly relevant for inter-RAT cell switches.
Both these problems imply that the mobility state of a device may not always be adequate, and thus appropriate scaling for the cell switch parameters is not always performed. Hence, there is a risk of e.g. connection drops, out-of-service, and unnecessary inter-RAT cell switches.
Therefore, there is a need for improved mobility management.